1. Field of the Invention
The present invention relates to an exchange coupling film for fixing a magnetization direction of a ferromagnetic body, a magnetoresistance effect device incorporating the same which causes a substantial magnetoresistance change with a low magnetic field, a magnetoresistance head incorporating the same which is suitable for use in high density magnetic recording and reproduction, and a method for producing such a magneto-resistance effect device.
2. Description of the Related Art
In recent years, the density of a hard disk drive has been dramatically increased, while a reproduction magnetic head has also been improved dramatically. Among others, a magnetoresistance effect device (hereinafter, referred to simply as an "MR device") utilizing a giant magnetoresistance effect, which is also called a "spin valve", has been researched actively and is expected to have the potential to significantly improve the sensitivity of a currently-employed magnetoresistance effect head (hereinafter, referred to simply as an "MR head").
An MR device includes two ferromagnetic layers and a non-magnetic layer interposed between the ferromagnetic layers. The magnetization direction of one of the ferromagnetic layers (hereinafter, referred to also as a "fixed layer") is fixed by an exchange bias magnetic field from a magnetization rotation suppressing layer (the ferromagnetic layer and the magnetization rotation suppressing layer are referred to collectively as an "exchange coupling film"). The magnetization direction of the other one of the ferromagnetic layers (hereinafter, referred to also as a "free layer") is allowed to change relatively freely in response to an external magnetic field. In this way, the angle between the magnetization direction of the fixed layer and that of the free layer is allowed to change so as to vary the electric resistance of the MR device.
An MR device has been proposed which utilizes NiFe for the ferromagnetic layer, Cu for the non-magnetic layer and Fe--Mn for the magnetization rotation suppressing layer. The MR device provides a magnetoresistance rate of change (hereinafter, referred to simply as an "MR ratio") of about 2% (Journal of Magnetism and Magnetic Materials 93, p. 101, (1991)). When Fe--Mn is used for the magnetization rotation suppressing layer, the resulting MR ratio is small, and the blocking temperature (a temperature at which the magnetization rotation suppressing layer provides no effect of fixing the magnetization direction of the fixed layer) is not sufficiently high. Moreover, the Fe--Mn film itself has a poor corrosion resistance. In view of this, other MR devices have been proposed which employ magnetization rotation suppressing layers with materials other than Fe--Mn.
Among others, an MR device which employs an oxide, such as NiO or .alpha.-Fe.sub.2 O.sub.3, for the magnetization rotation suppressing layer is expected to realize a dramatically large MR ratio of about 15% or greater.
However, the blocking temperature of NiO is not sufficiently high. Therefore, the thermal stability of the MR device employing NiO is undesirable.
When an MR device employs a magnetization rotation suppressing layer of .alpha.-Fe.sub.2 O.sub.3, on the other hand, the reverse magnetic field of the fixed layer is not sufficiently large when the magnetization rotation suppressing layer is thin. Particularly, an MR device having a dual spin valve structure or an MR device where an .alpha.-Fe.sub.2 O.sub.3 layer is formed on the fixed layer has a strong tendency that the reverse magnetic field of the fixed layer obtained in the overlying .alpha.-Fe.sub.2 O.sub.3 layer is insufficient. Moreover, the thermal stability of the .alpha.-Fe.sub.2 O.sub.3 -employing MR device is also undesirable for the same reasons as the NiO-employing MR device. Furthermore, the .alpha.-Fe.sub.2 O.sub.3 -employing MR device has other problems in controlling the anisotropy during deposition in a magnetic field or during a heat treatment in a magnetic field.